N- and O-linked glycans are found on both cell-surface and secreted proteins, many of which control proliferation and cell fate decisions in animals. Tissue-specific expression of glycosyltransferases is a significant factor controlling the glycan profiles observed in differentiated cells (Paulson, Jc and  Colley K J, J Biol Chem 1989, 264:17615-17618). In addition, many glycosyltransferases compete for acceptor intermediates causing bifurcations of the pathways and additional structural complexity (Schachter H, Biochem Cell Biol 1986, 64:163-181).
Specific glycan structures regulate lymphocyte adhesion, re-circulation and maturation as demonstrated by the GDP-fucose deficiency in LADII patients (11), and immune defects associated with C2 GlcNAc-T(L) (12) or ST3Gal-I (13) mutant mice. Depletion of the β1,6N-acetylglucosaminyltransferase V (Mgat5) modified glycans by swainsonine, an inhibitor of α-mannosidase II, potentiates antigen-dependent T cell proliferation, however, the molecular basis of this effect is unknown (14). Mgat5 catalyzes the addition of β1,6GlcNAc to N-glycan intermediates found on newly synthesized glycoproteins transiting the medial Golgi (15) (FIG. 1A). The glycans are elongated in trans-Golgi to produce tri (2,2,6) and tetra (2,4,2,6) antennary N-glycans which are preferentially extended with N-acetyllactosamine (Galβ1GlcNAc) and polymeric forms of N-acetyllactosamine also known as polylactosamine (6).